Download Decision Making for Food Choice by Grasshoppers

PLANT-INSECT INTERACTIONS
Decision Making for Food Choice by Grasshoppers
(Orthoptera: Acrididae): Comparison Between a Specialist Species
on a Shrubby Legume and Three Graminivorous Species
F. PICAUD,1 E. BONNET,1 V. GLOAGUEN,2
AND
D. PETIT
Faculté des Sciences, E.A. 3176, 123, av. A. Thomas, 87060 Limoges, France
Environ. Entomol. 32(3): 680Ð688 (2003)
ABSTRACT Dwarf gorse bush (Ulex minor) heathlands in Limousin, France, are ecological islands
often separated by tens of kilometers of grasslands and hedges, where several species of grasshoppers
belonging to the genus Chorthippus (Acrididae: Gomphocerinae) coexist. Chorthippus binotatus
(Charpentier) feeds only on Ulex minor; nymphs feed exclusively on leaves whereas adults become
ßorivorous at the end of the season. The other species studied (C. biguttulus (L.), C. albomarginatus
(De Geer), and C. parallelus (Zetterstedt)) are all graminivorous. The importance of sugars, nitrogen
content, sparteine (a quinolizidine alkaloid), and plant architecture in food selection was investigated.
Chorthippus binotatus is sensitive to sucrose and fructose, consistent with the high sugar content of
Ulex minor ßowers. Experiments with grass coated with sparteine showed that this molecule associated
with sucrose is a phagostimulant for this grasshopper. Different behavioral responses of graminivorous
species are observed with sparteine alone, but never phagostimulation. We compared the response
times corresponding to decision-making between the different species toward several components
involved in food selection. The food choice toward host plant and sugars is as quick for C. binotatus
as for the two graminivorous species (C. parallelus and C. albomarginatus), whereas C. biguttulus is
slower and exhibits atypical reactions. Chorthippus binotatus can feed on Poaceae, but with more time
spent, leading to an increasing predation risk. This situation is a limitation toward dispersal between
different heathlands (patchy habitats).
KEY WORDS decision making, patchy habitats, alkaloids, Chorthippus, diet shift, graminivory
CHAPMAN (1982) NOTED that in the majority of phytophagous insect orders, ⬎50% of the species are oligophagous (they feed on one plant family only) or
monophagous. The Orthoptera, and particularly Acridoidea, stand apart from the other orders: 60% of
grasshoppers have been classiÞed as polyphagous, and
a further 25% are graminivorous.
In North America, at least two Gomphocerinae species have evolved specialized diets from graminivory
under conditions in which grasses were often very
sparse (Otte and Joern 1977). Ligurotettix coquilletti
McNeill feeds on Larrea (Zygophyllaceae), Atriplex
(Chenopodiaceae), and Lycium (Solanaceae) (Chapman and Joern 1990). Bootettix argentatus Bruner
lives and feeds on the creosote bush Larrea tridentata
(Zygophyllaceae) (White and Chapman 1990). The
basis for this preference depends partly on the presence of the lignan nordihydroguaiaretic acid (NDGA)
(Chapman et al. 1988).
In France, the genus Chorthippus (Gomphocerinae) comprises 16 species (Bellmann and Lucquet
1993). The diet of all species was characterized by
1
2
Société Entomologique du Limousin, Limoges, France.
Laboratoire CSN, Faculté des Sciences de Limoges, France.
Gangwere and Morales-Agacino (1973). All species
are graminivorous with the exception of Chorthippus
binotatus (Charpentier) (Picaud et al. 1999). This species is present in France, Spain, and Morocco and
comprises six subspecies, Þve of which occur in France
(Defaut 1999). Chorthippus binotatus binotatus feeds
almost exclusively on various species belonging to the
Genisteae tribe (Fabaceae) (Grassé 1929), with some
regional specialization: Ulex minor Roth in Périgord
and Limousin, Ulex europaeus L. and Cytisus scoparius
(L.) Link in Charente-Maritime, Genista scorpius (L.)
DC. In the French Pyrenees (Defaut 1999), and Ulex
parviflorus Pourret in Vaucluse (Mossot and Petit
2000). In Limousin, the dietary preference of C. binotatus binotatus during the season is synchronized with
the life history of Ulex minor: the grasshopper feeds
shoots before blooming and ßowers during the summer (Picaud et al. 1999, 2002). In the laboratory, the
entire life cycle can be completed with Ulex minor as
the only food supply.
It is certain that C. binotatus evolved from graminivorous species: the 15 other species in the genus
Chorthippus feed on Poaceae, as well as the French
species of the sister genera Omocestus and Stenobothrus (Bonnet 1996). It is well known that grass-feeding
0046-225X/03/0680Ð0688$04.00/0 䉷 2003 Entomological Society of America
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Fig. 1. Distribution map of Chorthippus binotatus subspecies binotatus in France (hatched parts) and in Limousin Region
(open circles), divided in three departments (after Kruseman, 1982; Defaut, 1999). Plant samples for analyses were collected
in Ceinturat heathland (black circle).
grasshoppers feed mainly on the most abundant
Poaceae species (Bernays and Chapman 1994). Because the chemical composition of secondary metabolites of Graminae are rather uniform (Bruneton
1999), one can assume that grass-feeding grasshoppers
are oligophagous and therefore relatively specialist
species.
In Limousin, the heathlands correspond to ecological islands (Brugel et al. 2001). Thus, for species using
food resources that are limited and patchy, the distance between these bush formations can be a constraint for dispersal. A less obvious constraint that may
be associated with oligophagy is the problem of food
quality, including nitrogen and/or carbohydrates levels, and the balance between deterrents and phagostimulants.
Bernays and Bright (2001) showed that food choice
of Schistocerca americana (Drury) between different
combinations of two plant species causes interrupted
feeding as a result of inefÞcient decision making. So,
feeding on only one plant species saves time and potentially beneÞts grasshoppers. One can ask several
questions about the properties linked to feeding upon
a given plant: (1) in the case of binary choice (Ulex
minor versus Poaceae), do the insects feed exclusively
on their host plants; (2) what are the characteristics of
Ulex minor involved in the diet preference of C. binotatus; and (3) is the specialist species on Genisteae
bushes as efÞcient as each graminivorous species in
their food selection?
In this work, we examined the feeding behavior and
the food selection of all the Chorthippus species coexisting with Chorthippus binotatus in Limousin heathlands: C. biguttulus (L.) (subgenus Glyptobothrus, as
C. binotatus), C. parallelus (Zetterstedt), and C. albomarginatus (De Geer) (subgenus Chorthippus). It is
known that the three latter species are graminivorous
(Bonnet 1996). We investigated food quality as the
basis of food selection in grass feeding and Ulex minor
specialist grasshoppers. We analyzed sugars and nitrogen content in the plant species with a special focus
on Ulex minor and Poaceae. As for secondary compounds, it is well known that numerous quinolizidine
alkaloids characterize the tribe of Genisteae (Bruneton 1999). Our investigation was limited to sparteine,
the major alkaloid in the genus Ulex (Picaud et al.,
unpublished data). The inßuence of plant architecture was also tested: ramiÞed in the case of Ulex,
straight in the case of Poaceae.
We discuss the Þndings in relation to the beneÞt of
quick decision times (Bernays and Bright 2001,
Abrams and Schmitz 1999) and the disadvantage of
being a specialist on Ulex minor, as the Ulex minor
heathlands form islands between grasslands and pastures.
Materials and Methods
Experimental Animals. Chorthippus adults (C. binotatus, C. biguttulus, C. albomarginatus, and C. parallelus) were collected in September and October 1993Ð
2001 in heathlands in the Limousin region (45Ð 46
degrees N. latitude, 1Ð2 degrees E. longitude), France
(Fig. 1). If we take into account the mean of absolute
plant cover in seven heathlands where C. binotatus
was present, the most abundant plant species are: Ulex
minor (48%), Calluna vulgaris (34%), Molinia caerulea
(23%), Erica cinerea (17%), Pteris aquilinum (7%), and
Erica tetralix (4%). Insects were placed individually in
glass jars (6 cm diameter ⫻ 10 cm height) and kept at
room temperature (20 ⫾ 2⬚C), under a L:D 12:12 h
photoregime and 50% humidity for the duration of the
experiments. They were fed daily with fresh plants
collected at the Limoges University: Ulex minor for
C. binotatus and a mix of Agrostis tenuis, Holcus lanatus,
and Lolium perenne for the graminivorous species.
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Every week after the experiments, all living insects
were returned to the wild and replaced by newly
collected specimens.
Tested Plants and Food Selection Experiments. We
chose Ulex minor (ßowering time in JulyÐOctober)
and the most abundant grass in heathlands (Molinia
caerulea) for testing. Another grass species, Lolium
perenne, was also tested because of its abundance at
the Limoges University. Before each experiment, food
was withheld from insects for 4 Ð 6 h (Bernays and
Chapman 1970).
Food choice tests with 16 Ð32 individual adults of
both sexes of each species were performed in a Petri
dish (14.2 cm diameter ⫻ 2 cm height) isolated in a
green enclosure. A 40 W-lamp above the center of the
dish provided constant light and minimized temperature ßuctuations. Each experiment was limited to
15 min to avoid postingestive feed-backs. Thus, decisions by grasshoppers depended on receptor sensitivity and the physiological state of individuals. The
grasshopper was transferred to the center of a dish in
a small glass jar. It was given a choice between two
separate sources, each consisting of fresh plants
(⬇1 g), cut with scissors just before the start of the
bioassay. Three different behavioral responses were
recorded (Williams 1954, Bernays and Chapman 1970,
Uvarov 1977): orientation (the grasshopper walked to
the plant (item 1), palpated the plant (item 2), and ate
the plant (item 3). To help understand food choice, we
examined sugars, architecture, and the secondary metabolite sparteine.
During plant choice experiments, the sum of orientation and palpation times from initiation of movement until the decision (feeding upon or rejecting it)
was documented. Concerning the experiments with
sugars and the alkaloid, non - volatile chemical compounds, only palpation time before decision was taken
into account.
Chemical Analysis of Plants (soluble carbohydrates
and nitrogen content). The samples of Ulex minor
ßowers, shoots, and stems were collected from Ceinturat heathland (Fig. 1), Þve in September 1997 and
four in July 1999 at the beginning of blooming for a
series of analyses. Four samples of two species of
Graminae were collected in grasslands of the Limoges
University in early July 1999. The separation and identiÞcation of soluble sugars (glucose, fructose and sucrose) was adapted from Harisson et al. (1997) and
Cataldi et al. (1999) and performed by high performance liquid chromatography (HPAEC-PAD). Elution of the samples was carried out in isocratic conditions by using 0.16 M aqueous NaOH at a ßow rate
of 1 ml ⫻ min⫺1. Calculations of mono- and disaccharide concentrations were made from dry weight extraction residues, and then expressed relative to initial
sample dry weight. The measurement precision was
tested by duplicating analyses of the samples.
The nitrogen content was evaluated by Kjehldahl
analysis (AOAC 1984).
Sugar Sensitivity. The experimental apparatus was
the same as that described above, with the exception
that the length of the test was reduced to 5 min to
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Fig. 2. Drawings presented to insects in plant architecture experiments.
avoid consumption of the piece of Þlter paper. The
choice consisted of two pieces of Þlter paper (each
one measuring 6 ⫻ 6 cm, and four folds), placed at
random on opposite edges of the dish, one with one M
sugar solution (1 ml), and one with distilled water
(20 Ð21 individuals for each species).
Decomposition Analysis of Shape of the Plant. Individual grasshoppers were placed for 10 min in a
Y-maze with opaque walls (30 ⫻ 8 ⫻ 11 cm) covered
with Plexiglas. The grasshoppers had to choose between two drawings (8 ⫻ 11 cm) placed vertically and
at random ends of the two branches (18 ⫻ 8 ⫻ 11 cm).
Choice between the drawings was recorded when the
insect reached the end of the Y-maze and climbed on
the drawing. The time from initiation of movement
until climbing onto the drawing was recorded. Two
pairs of green patterns on a yellow background, to
mimic Ulex branches in the environment, were presented: each individual (19 Ð21 by species) had to
choose between straight versus branching aspects and
straight versus crossed aspects in separate bioassays
(Fig. 2).
Alkaloid Sensitivity. Preliminary experiments
demonstrated (data not shown) that C. binotatus and
C. biguttulus have no preference or rejection behavior
for 1% acetic acid versus water, on Þlter paper. Alkaloid content in Leguminosae vary generally between
0.1 and 3% of plant dry weight (Bruneton 1999). Our
analyses of different organs of Ulex minor (Picaud et
al., unpublished data) have shown that sparteine represents between 30 Ð 48% of nonlinked alkaloids. Different concentrations (in 1% acetic acid) of sparteine
99% (Sigma Aldrich, Saint-Quentin, France) were
tested under the same conditions as described in
“sugar sensitivity” trials, with a 1% acetic acid treatment as a control. Experiments were repeated three
times with different sets of 16 Ð24 individuals of
C. binotatus, tested separately with 0.1%, 0.5%, 1% and
2% of sparteine. The principal behavioral response
recorded was the arrestment on the Þlter paper.
InterspeciÞc comparisons of behavioral responses
toward sparteine were undertaken with a solution of
1% sparteine in 1% acetic acid on Þlter paper in the
center of a glass container (5 cm diameter at base, 7 cm
high), with a 1% acetic acid treatment as a control. For
each species, we observed 20 Ð24 separated individuals. For each individual, we recorded 10 items during
a 5-min continuous observation. The behaviors were:
palpation in the air (Pair); palpation of Þlter paper
(Palp); arrestment on the Þlter paper (Arrest); put-
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683
Fig. 3. InterspeciÞc comparison of three behaviors of Chorthippus binotatus (bin), C. biguttulus (big), C. albomarginatus
(alb), and C. parallelus (par) in binary food plant experiments. (a) Results (mean ⫾ SEM of differences) from Wilcoxon test
on behaviors (orientation: 1, palpation: 2, consumption: 3) for plant choice. Kruskal-Wallis tests of all combined data are noted
on each Þgures. Different letters correspond to signiÞcant differences between species (MannÐWhitney U tests, P ⬍ 0.05).
(b) Mean reaction times ⫾ SEM (seconds, Log10-transformed data) to move and palpate the plant. ANOVA of the combined
data of all species concerning reaction time for ßowered Ulex minor: F3,27 ⫽ 3.62, P ⫽ 0.026. ANOVA for Lolium perenne: F3,30 ⫽
17.16, P ⬍ 0.001. Different letters correspond to signiÞcant differences between species (TukeyÕs test, P ⬍ 0.001).
ting the fore legs on the Þlter paper and touching the
mouth parts (Leg); putting the labrum on the Þlter
paper (Lab); biting and masticating (Bmas); leg withdrawing above the Þlter paper (Wleg); regurgitation
(Reg); leaving Þlter paper (Leave); jumping resulting
from Þlter paper contact (Jump).
We tried to enhance grass consumption by C. binotatus in adding some characteristic molecules present
in Ulex minor: we studied its behavior with Lolium
perenne covered, with a brush, with different combinations of solution diluted in 1% acetic acid: sucrose
one M, sparteine 1%. Lolium perenne covered with 1%
acetic acid were used as control. We recorded the time
spent palpitating before decision and the frequency of
individuals (16 Ð22 individuals for each experiment)
consuming Lolium perenne.
Statistical Tests. The individual behaviors for each
choice were analyzed with Wilcoxon tests. Differences given by these tests were used to perform
Kruskal-Wallis tests for interspeciÞc comparisons using SYSTAT ver. 7.0 (SPSS 1997).
The reaction times were compared using a multiway analysis of variance (ANOVA) because all of the
data were normally distributed. Log-transformed palpation times were compared using multi-way ANOVA
for interspeciÞc comparisons. In each case, TukeyÕs
tests were performed for pair-wise comparisons.
Frequencies of different items with sparteine trials
and coated grass experiments were compared with a
MannÐWhitney U test.
Results
Behavioral Comparison among Species. Chorthippus binotatus and C. biguttulus preferred Ulex minor
ßowers (Fig. 3a) to Lolium perenne (Kruskal-Wallis
test: H3 ⫽ 18.166, N ⫽ 88, P ⬍ 0.001) and Molinia
caerulea (Kruskal-Wallis test: H3 ⫽ 16.30, N ⫽ 79, P ⫽
0.001). When the Ulex minor branches have not ßowered yet, the graminivorous species (C. biguttulus,
C. albomarginatus, and C. parallelus) preferred Graminae (Kruskal-Wallis test: H3⬍10.96, 74⬍N⬍79, P ⬍
0.015 with both binary plants choice).
The shortest time (Fig. 3b) between the beginning
of the experiment and the palpation of Ulex minor in
the case of binary choices was observed for C. binotatus (ANOVA: F3,27 ⫽ 3.62, P ⫽ 0.026). C. albomarginatus and C. parallelus show the quickest reaction
time to Lolium perenne (ANOVA: F3,30 ⫽ 17.16, P ⬍
0.001).
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ENVIRONMENTAL ENTOMOLOGY
Vol. 32, no. 3
Fig. 4. InterspeciÞc comparison of three behavioral items of Chorthippus binotatus (bin), C. biguttulus (big), C. albomarginatus (alb), and C. parallelus (par) in an experiment testing the response to different sugars versus water. (a) Mean ⫾
SEM of differences from Wilcoxon test on behaviors (orientation: 1, palpation: 2, consumption: 3). Kruskal-Wallis test of all
combined data are noted on each Þgures. Different letters correspond to signiÞcant differences between species (MannÐ
Whitney U tests, P ⬍ 0.05). (b) Mean palpation time ⫾ SEM (seconds, log-transformed data) of soluble carbohydrate solution
(1M). ANOVA of the combined data of all species for each sugar experiment: see text. Different letters correspond to
signiÞcant differences between species (TukeyÕs test: P ⬍ 0.05).
The most marked preference for sucrose (Fig. 4a)
was observed for C. binotatus (Kruskal-Wallis test:
H3 ⫽ 10.49, N ⫽ 80, P ⫽ 0.015). This result is corroborated by the fact that C. binotatus showed the quickest palpation time (Fig. 4b) before making a decision
in the presence of sucrose (ANOVA: F3,28 ⫽ 3.91, P ⫽
0.019). Curiously, C. biguttulus responded slowly to
sucrose. As for fructose, the most complete series of
behaviors was performed by C. binotatus and C. parallelus (Kruskal-Wallis test: H3 ⫽ 7.56, N ⫽ 82, P ⬍
0.056). Unexpectedly, C. biguttulus had the quickest
reaction time (Fig. 4b, ANOVA: F3,31 ⫽ 3.53, P ⫽ 0.026)
in spite of its low motivation for fructose (Fig. 4a).
C. biguttulus is the only species sensitive to glucose
(Kruskal-Wallis test: H3 ⫽ 10.23, N ⫽ 81, P ⫽ 0.017),
but no difference between decision time was recorded
(ANOVA: F3,29 ⫽ 0.77, P ⫽ 0.52).
Finally, C. binotatus was the only species attracted
(Fig. 5) by branching aspects (Kruskal-Wallis test:
H3 ⫽ 13.41, N ⫽ 82, P ⫽ 0.004).
Soluble Carbohydrates and Nitrogen Contents.
Flowers of Ulex minor had high levels of sucrose
(mean ⫾ SEM ⫽ 18.9 ⫾ 0.7 mg ⫻ g⫺1 of extract
residue) in the autumn but not summer (Table 1).
Glucose and fructose were both present at very high
levels in Ulex minor (⬎58 mg 䡠 g⫺1 of extract residue).
Shoots of Ulex minor also contained fructose (mean ⫾
SEM ⫽ 3 ⫾ 1.4 mg 䡠 g⫺1) and sucrose (mean ⫾ SEM ⫽
Fig. 5. InterspeciÞc comparison of orientation behavior
of Chorthippus binotatus (bin), C. biguttulus (big), C. albomarginatus (alb), and C. parallelus (par) in a Y-maze. Results
(mean ⫾ SEM of the differences) of Wilcoxon test on orientation behaviors an experiment comparing different plant
architecture. Kruskal-Wallis test of all combined data are
noted on each Þgures. Different letters correspond to signiÞcant differences between species (Mann-Whitney U tests,
P ⬍ 0.05).
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685
Table 1. Determination of soluble carbohydrates by HPAEC-PAD analyses and nitrogen content by Kjehldahl analysis. Sugars and
nitrogen (mean ⴞ SEM) are expressed in mg.gⴚ1 for dry weight. Analyses were performed on 500-mg plant tissues of 5 samples of
Ulex minor and 4 samples of each grass
Flowers of Ulex minor (end of July)
Flowers of Ulex minor (September)
Stems of Ulex minor
Shoots of Ulex minor
Leaves of Agrostis tenuis
Leaves of Holcus lanatus
Glucose
Fructose
Sucrose
Nitrogen
58.3 ⫾ 0.9
69.1 ⫾ 0.5
5.6 ⫾ 2.1
3.95 ⫾ 0.8
7 ⫾ 2.5
4.2 ⫾ 1.7
67.7 ⫾ 6.7
77.4 ⫾ 0.9
2.6 ⫾ 0.4
3 ⫾ 1.4
8.4 ⫾ 4.1
3.6 ⫾ 2
3.5 ⫾ 0.2
18.9 ⫾ 0.7
8 ⫾ 0.8
6.6 ⫾ 0.8
9.6 ⫾ 0.9
8.7 ⫾ 1
5.16 ⫾ 0.5
No data
9.6 ⫾ 1.2
9.8 ⫾ 0.6
No data
No data
6.6 ⫾ 0.8 mg 䡠 g⫺1), representing ⬍10% of the quantity
present in ßowers. The two species of grasses contained sugars in comparable amounts to stems of
Ulex minor (Table 1). Ulex minor ßowers were low in
nitrogen content (0.5Ð 0.6% of dry weight). Nitrogen
made up ⬇1% of the dry weight of shoots and stems
of Ulex minor.
Sparteine Sensitivity. At concentrations between
0.5 and 2% sparteine, 25% of C. binotatus individuals
were arrested by the piece of Þlter paper. For weaker
concentrations, the percentages of responses clearly
fall (12% of individuals for 0.1% sparteine). Consequently, the sparteine concentration used in interspeciÞc experiments was 1%.
Among the 10 behaviors considered, four were of
special interest because of statistical differences between species (Fig. 6): air palpation, arrestment on the
piece of Þlter paper, leaving Þlter paper and regurgi-
tation. It should be noticed that except one individual
of C. albomarginatus, arrestment and leaving Þlter
paper were never associated. C. binotatus differed
from remaining species by three behaviors when
sparteine is dropped on Þlter paper. C. binotatus palpated the air more than the other species (KruskalWallis test: H3 ⫽ 18.94, N ⫽ 57, P ⬍ 0.001). Arrestment
on the piece of Þlter paper characterized C. binotatus
(Kruskal-Wallis test: H3 ⫽ 16.27, N ⫽ 57, P ⫽ 0.001).
In the same way, C. binotatus was the only species
never leaving Þlter paper (Kruskal-Wallis test: H3 ⫽
19.03, N ⫽ 57, P ⬍ 0.001).
Otherwise, regurgitation characterized C. albomarginatus compared with other species (Kruskal-Wallis
test: H3 ⫽ 21.12, N ⫽ 57, P ⬍ 0.001).
Within each species, we tested to determine
whether there was a signiÞcant difference between
the frequencies of each behavior in response to acetic
Fig. 6. Frequency of different behaviors in response to 1% sparteine diluted in 1% acetic acid on Þlter paper (shaded
boxes) or to 1% acetic acid alone (open boxes). Asterisks indicate signiÞcantly different values from all other species tested
(Kruskal-Wallis test, P ⱕ 0.001 in each case). The circles indicate signiÞcant differences between frequencies of response
to sparteine and the acetic acid control (Mann-Whitney U test, P ⬍ 5%) in each case), when both frequencies are not null.
In the case of acetic acid alone, the behavior “no-response” is not represented. Pair: palpation of air; Palp: palpation of Þlter
paper; Arrest: arrestment on Þlter paper; Leg: placing fore legs on Þlter paper and touching with mouth parts; Lab: touching
Þlter paper with labrum; Bmas: biting and masticating; Wleg: withdrawing leg above Þlter paper; Reg: regurgitation; Leave:
leaving Þlter paper; Jump: jumping in response to contact with Þlter paper.
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Vol. 32, no. 3
acid versus sparteine diluted in acetic acid. The responses of C. binotatus are generally stronger when
sparteine is present (MannÐWhitney U test, P ⬍ 0.01
in each case), except for biting behavior. The behaviors related to avoidance (leaving Þlter paper, jumping, leg withdrawing, regurgitation) are generally expressed in graminivorous species when sparteine is
present. Curiously, C. parallelus is the only graminivorous species palpitating Þlter paper with sparteine at
a higher frequency than acetic acid (MannÐWhitney
U test, P ⬍ 0.05).
Responses of C. binotatus toward Lolium perenne
Coated with Different Combinations of Solutions.
The time spent on palpation of L. perenne before
decision (consumption or rejection) is shown in Fig.
7a. The shortest time, i.e., the quickest decision corresponded to the combination of sucrose and
sparteine. This time differs signiÞcantly from the one
recorded with L. perenne coated with acetic acid only
(ANOVA: F1,26 ⫽ 5.97, P ⫽ 0.02).
Consumption frequency of Lolium perenne by
C. binotatus (Fig. 7b) was enhanced by the same
combination of sparteine ⫹ sucrose (Kruskal-Wallis
test: H3 ⫽ 8.24, N ⫽ 69, P ⫽ 0.04). The effect of addition
of sucrose or sparteine alone did not signiÞcantly differ from the test.
Discussion
Analysis of Food Choice. Chorthippus binotatus prefers Ulex minor, especially ßowers. Both C. albomarginatus and C. parallelus prefer Graminae to Ulex,
whether blooming or not. Our experiments lead us to
deÞne three feeding groups. The Þrst one is represented by C. binotatus, which specializes on Ulex minor, and the second one comprises C. albomarginatus
and C. parallelus, which are “purely graminivorous.”
The third group consists of C. biguttulus, which behaves like the purely graminivorous species when Ulex
is not ßowering, but like the specialist species on
Genisteae when Ulex is ßowering. We can call this
species “atypically graminivorous.” Thus, both C. binotatus and C. biguttulus respond to Ulex minor ßowers,
which probably contain one or more attractants.
These observations are conÞrmed by measuring
time to decision making. The two purely graminivorous species have the quickest reaction time for
Graminae and the longest one for Ulex minor, whereas
the specialist species on Genisteae shows the opposite
behavior. Curiously, the “atypical graminivorous species” (C. biguttulus) is slow to make a decision, regardless of its choice (Ulex minor or Graminae) and
has the broadest diet, accepting Ulex as well as grasses.
This agrees with the Þndings of Bernays (1998, 2001)
that the broader the food types accepted, the slower
the response (assuming grasses to be rather uniform in
secondary chemical compounds). This disadvantage is
somewhat balanced by the fact that C. biguttulus has
a larger habitat niche than either of the other species
(Bellmann and Luquet 1993).
Chorthippus binotatus is the species most responsive to sucrose, and it is as responsive to fructose as
Fig. 7. Behavior of Chorthippus binotatus in response to
Lolium perenne covered with different combinations of solutions diluted in acetic acid 1% (sucrose: 1 M, sparteine: 1%).
(a) Mean time ⫾ SEM spent (seconds, log-transformed data)
in palpation (global ANOVA: F3,52 ⫽ 3.41, P ⬍ 0.05). Different
letters correspond to signiÞcant differences between treatments (TukeyÕs test: P ⬍ 0.05). (b) Consumption frequency.
Combined data of each combination are compared with
Kruskal-Wallis test: H3 ⫽ 8.24, N ⫽ 69, P ⬍ 0.05). Different
letters correspond to signiÞcant different frequencies between treatments (Mann-Whitney U tests, P ⬍ 0.05).
C. parallelus. Chorthippus biguttulus is most sensitive
to glucose. As a result, sucrose and fructose are probably the major feeding stimulants, among sugars, on
Ulex minor. The sugar analyses show that Ulex minor
ßowers in September had the highest content of sugars. However, the attraction of C. binotatus to its host
plant in July is not explained by sugar content.
Presence of receptors that can detect sugars has
been demonstrated in numerous insects (Mitchell
1981) and especially in grasshoppers (Bernays and
Chapman 1994): the pyranose site is maximally sensitive to sucrose and glucose and the furanose site to
fructose. Feeding on Ulex minor ßowers is partly explained by sensitivity of C. binotatus to sucrose and of
June 2003
PICAUD ET AL.: DECISION MAKING FOR FOOD CHOICE BY GRASSHOPPERS
C. biguttulus to glucose. Fructose does not play any
role as the “strictly graminivorous” species are not
attracted by Ulex minor ßowers.
Sparteine alone leads to different behavioral responses according to the species. Leaving the substrate containing sparteine is a response characteristic
for graminivorous species. C. albomarginatus regurgitates in response to sparteine. Contrary to the other
species, C. binotatus palpates the air and remains on
the substrate. Thus, sparteine is at least involved in the
food selection of the insect species enabling it to feed
on Genisteae. This situation can be compared with the
phagostimulant power of pyrrolizidine alkaloids to
Zonocerus elegans Thunberg (Boppré et al. 1984), although it is a polyphagous and opportunistic species.
Our study on C. binotatus with Lolium perenne
coated with different solutions showed that the combination of sparteine and sucrose is probably involved
in food choice of C. binotatus. This is in agreement
with our results that the highest consumption level
was recorded in presence of the same combination,
demonstrating its role as a phagostimulant.
With respect to the plant architecture, we found
that C. binotatus was attracted to the branchy drawing
over the straight one. This can partly explain its preference to bushes versus grasses. However, plant architecture cannot be the only explanation for the
feeding behavior of this species, because it is never
attracted to other ßowering bushes in heathlands, e.g.,
Calluna vulgaris, Erica tetralix, and Ulex europaeus.
Contrary to the results of Williams (1954) and Bernays
and Chapman (1970), graminivorous species were not
signiÞcantly attracted to a given architecture in our
experiments. These authors used only three-dimensional lures and never drawings.
Benefits and Disadvantages of Being a Specialist on
Fabaceae Bushes. Chorthippus binotatus has evolved
from graminivorous species. We hypothesize that the
shift could have been the consequence of an evolved
sensibility to sparteine (and/or other quinolizidine
alkaloids) in the shoots and not to Genisteae ßowers
because (1) young instars of the subspecies C. b. binotatus are never exposed to ßowers, whereas (2) the
adults of subspecies C. b. moralesii feed on both grasses
and nonßowering Genisteae (Picaud et al. 1999).
However, the diet shift within the genus Chorthippus
has ecological consequences on nutrient balance, dispersal, and possibly predation risk.
Grasses growing on rich soils in spring contain as
much nitrogen as Genisteae shoots and stems. However, nitrogen concentration of grasses decreases with
plant maturity (Chapuis and Lefeuvre 1980). Finally,
Genisteae in the autumn are richer in nitrogen than
grasses growing on the poor soils of heathlands (Rozé
1980, Glyphis and Puttick 1989). Consequently, feeding on Genisteae represents a gain in terms of nitrogen
supply, at least when Ulex minor shoots and stems are
consumed. However, when the specialist species shifts
its diet toward U. minor ßowers, we assume that the
nitrogen supply falls. If compensatory feeding occurs,
C. binotatus should eat twice as much on ßowers as
shoots. Nevertheless, the major advantage of this shift
687
should be an increase of Þtness because of the presence of carbohydrates. We hypothesize that the sugars
present in ßowers favor the reproductive investment
that occurs from the end of August until October, i.e.,
the main ßowering season.
Graminivorous species have no constraint for dispersal as grasslands and hedges are connected. The
occurrence of C. binotatus, however, is limited by the
patchy distribution of Ulex minor heathlands. However, we showed that this specialist species can feed on
Poaceae when there is no other available resource.
Guéguen et al. (1975) found Poaceae phytolithes in
C. binotatus faeces in Brittanny. It should be pointed
out that when the species specializing on Genisteae
feeds on Poaceae, its vigilance level probably decreases, leading to a predation risk (Dukas 1998).
Thus, population migrations between relatively close
heathlands are possible but certainly infrequent. It
would be interesting to know the percentage of individuals able to complete their life cycle with Poaceae
as the only food source. Our preliminary experiments
(Picaud et al. 2002) suggest that this percentage is very
low at best. If a few generations could develop on
grass, it would allow the species to colonize distant
heathlands islands. Genetic distances between populations should be measured to estimate gene ßow to
address whether these populations are part of one or
several metapopulations, and thus the extinction risk
of each population.
Acknowledgments
We are grateful to Cécile Nouhaut for her help in translation, and A. Bellet, S. Verot, and I. Vannier for their technical assistance.
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Received for publication 1 April 2002; accepted 18 February
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